The invention relates especially to an improved method for the preparation of methoxymethylene compounds of acetic acid methyl ester, which can be substituted if desired, and of ethoxymethylene compounds from unsubstituted and substituted acetic acid ethyl esters. In particular, methoxymethylene compounds of malonic acid dimethyl ester, and also of cyanoacetic acid methyl ester, methoxyacetic acid methyl ester and acetic acid methyl ester, are prepared by this method.
Accordingly, ethoxymethylene compounds especially of malonic acid diethyl ester, and also of cyanoacetic, methoxyacetic, ethoxyacetic and acetic acid ethyl esters can be prepared. If needed, alkoxymethylene compounds of the above-named esters can also be prepared, which have saturated aliphatic alkoxy groups of 1 to 6 carbon atoms, and which very preferentially have in the ester groups the same saturated or unsaturated alkyl moieties of 1 to 6 carbon atoms as in the alkoxymethylene group.
A special advantage of the process is that enol ethers can be prepared with hydroxymethylene compounds which are stable only as sodium salts, such as the sodium salts of hydroxymethylene acetic acid methyl ester and hydroxymethylene methoxyacetic acid methyl ester.
It is known to react the methoxymethylene compound of malonic acid dimethyl ester with trimethylorthoformate in the presence of acetic anhydride and zinc chloride (Claisen, A. 297 1 (1897), German Fed. publication AS No. 24 26 964). A disadvantage of the process is, however, that it requires long reaction times and a great amount of distillation if good yields are to be achieved.
In accordance with German Fed. publication OS No. 18 00 352, malonic acid dimethyl ester is reacted with carbon monoxide in the presence of sodium methylate and methanol to the sodium salt of hydroxymethylene malonic acid dimethyl ester, and this is transformed in an inert solvent, such as toluene, by reaction with dimethylsulfate to methoxymethylene malonic acid dimethyl ester. In addition to the high toxicity of the dimethylsulfate, the reaction has the disadvantage that only one methyl group of the dimethylsulfate is utilized. Alkohols instead of dialkylsulfat do not form alkoxymethylene compounds.
The problem therefore existed of preparing alkoxymethylene compounds, especially methoxymethylene compounds, from acetic acid esters and substituted acetic acid esters having 1 to 6 carbon atoms in the alkyl moiety of the ester group, especially from methyl esters, in a simple manner with a high yield.